Increased interest in global environmental issues has led to a demand for superior fuel consumption performance in pneumatic tires, along with high wet performance and superior safety performance. As a result, by compounding silica in rubber compositions that form tread portions, dynamic viscoelastic characteristics of the tread rubber such as loss tangent (tanδ) have been improved, wet performance has been improved while heat build-up has been suppressed, rolling resistance reduced, and fuel consumption performance improved. However, the particles of silica tend to aggregate together through hydrogen bonds of silanol groups, which are the surface functional group thereof, and affinity with diene rubber and dispersion tends to be poor, and the effects of improving low rolling resistance and wet performance are not sufficiently obtained.
For this reason, silica dispersibility in the rubber composition has been improved by compounding a silane coupling agent containing mercapto groups together with silica (for example, refer to Japanese Unexamined Patent Application Publication No. 2009-126907A). A mercapto group-containing silane coupling agent has high affinity with silica and is excellent in improving dispersibility, and improvements in rolling resistance and wet performance are seen. However, in a rubber composition in which a mercapto group-containing silane coupling agent is compounded, there is the problem that extrudability is poor, due to high viscosity and scorch (premature curing) readily occurring and the like, and even in Japanese Unexamined Patent Application Publication No. 2009-126907A, an attempt to improve viscosity and workability was made but the improvement were not necessarily sufficient.